1. Field of the Invention
This invention relates to a process for recovering shale oil from an in-situ share retort, and more particularly to a process for returning hydrogen sulfide and a portion of the carbon dioxide separated from the shale oil along with a molecular oxygen-supplying gas to the retort for oxidation to sulfur oxide compounds which then react with spent and/or retorted shale.
Shale deposits in the Western States of the United States extend over thousands of square miles and are more than a thousand feet thick in some areas. Shale contains kerogen, a solid carbonaceous material which on heating to a temperature above 427.degree. C. yields shale oil. The shale deposits can produce from about 15 to about 80 gallons of shale per ton of shale. By retorting is meant heating a carbonaceous material to about 427.degree. C. so as to produce liquid and gaseous product by cracking and distillation reactions.
One method of recovering shale oil from shale is to treat shale at high temperatures in retorts located at the ground surface. Because of the expense of underground mining of shale, it is preferred to retort shale in-situ. A problem that confronts in-situ retorting of shale is the disposal of harmful pollutant gases, such as, for example, hydrogen sulfide, carbonyl sulfide, mercaptans and carbon disulfide given-off during the combustion and/or retorting processes. Presently, shale technology generally calls for the recovery by complex and expensive technology of these pollutant gases to prevent their release into the atmosphere.
Green River shale contains from 0.6 to 0.7 percent by weight total sulfur (Stanfield, K. E., Frost, I. C., McAuley, W. S. and Smith, H. N., "Properties of Colorado Oil Shales," Bureau of Mines Report of Investigations 4825, 1951). A more detailed analysis of organic sulfur and pyrite sulfur present in Colorado shale is reported in RI 5725 (Smith, John Ward, "Ultimate Composition of Organic Material in Green River Oil Shale," Bureau of Mines Report of Investigations 4825, 1961). An average of 10 cored samples in Colorado and Utah oil shale showed a total sulfur (organic plus pyrite) of 0.63 percent by weight. The organic content of the cores averaged 14.1 percent, and this organic fraction contained 1.0 percent sulfur. Thus, 22 percent of the sulfur is organic sulfur and the remaining 78 percent is essentially inorganic sulfur.
"Revised Detailed Development Plan, Tract C-a Volume I," submitted to Area Oil Shale Supervisor Geological Survey, U.S. Department of the Interior, pp. i, iii and iv, describes a commercial plan for retorting 170,000 tons of shale per day which would produce 265 tons of sulfur per day. Some of the sulfur in the shale does not decompose and a portion ends up in the liquid product, but is is estimated that from 20 to 75 percent of the total sulfur present in the in-situ shale can end up in the gas.
Retorting with air produces fuel gases which are contaminated with the sulfur gases and, in addition, produces fuel gases which contain from 85 to 90 percent of inert gases such as, for example, nitrogen and carbon dioxide. Typical heating values of the gas are only about 35 to about 65 Btu/SCF, which makes such gases difficult to use in combustion processes or gas turbines except by resorting to supplemental fuel. If the fuel gas can be upgraded to about 80 to about 100 Btu/SCF, supplemental fuel is not needed. Carbon dioxide can be removed from gases to improve heating value, and sulfur gases can be concentrated and processed by surface processes to give elemental sulfur as a product. However, removal of hydrogen sulfide which is recovered as elemental sulfur and upgrading of the fuel gases by removal of carbon dioxide so it can be burned can be very complex and expensive. One process commonly used for removing hydrogen sulfide and carbon dioxide is called the hot potassium carbonate process which was developed by the U.S. Bureau of Mines for removing acid gases from coal synthesis gas.
It has been discovered that a selective process can first be employed to absorb substantially all of the hydrogen sulfide and a portion of the carbon dioxide using a selective hot carbonate process. In a preferred embodiment a portion of the hydrogen sulfide-free gas can then be cleaned substantially completely of carbon dioxide, and this carbon dioxide-free gas is blended with the hydrogen sulfide-free gas to give a fuel gas which can be satisfactorily burned whithout polluting the atmosphere. The recycling of the hydrogen sulfide-containing gas and the processing sequence for scrubbing the off-gases gives an optimum solution to a difficult problem.
Consequently, a need exists for a simpler process for removing pollutant gases, in particular, hydrogen sulfide. In accordance with the invention herein, an improved process is provided for recovering shale oil from in-situ shale comprising the steps of: (1) mining a first portion of said shale; (2) fragmenting a second portion of said shale; (3) introducing into said second portion a mixture of gases comprising a molecular oxygen-supplying gas, carbon dioxide and hydrogen sulfide while maintaining a temperature sufficient to convert kerogen in said second portion to shale oil and to produce carbon dioxide, hydrogen sulfide, gaseous hydrocarbons and other combustion and inert gases; (4) separating said shale oil from said carbon dioxide, hydrogen sulfide, gaseous hydrocarbons and other combustion and inert gases; (5) separating said hydrogen sulfide and a first portion of said carbon dioxide from said gaseous hydrocarbons and other combustion and inert gases and a second portion of said carbon dioxide; and (6) recycling said hydrogen sulfide and said first portion of said carbon dioxide to step (3).
2. Description of the Prior Art
Unlike the invention herein, U.S. Pat. No. 2,630,307 to Martin relates to a method of recovering oil from in-situ oil shale by destructively distilling oil shale using a combustion supporting gas containing carbon dioxide and oxygen in a critical ratio.